One of the most vexing problems that surgeons face when using surgical bone saws is the tendency of the saw to "kick"; i.e., to become caught upon the bone being cut by the point of the saw tooth. Another form of kicking occurs where the kerf has the same contour as the blade which is due to the fact that the cutting surface (i.e., teeth) lies on the same radius as the radius of tool rotation. In this scenario, all teeth grab nearly at the same time. This causes the saw apparatus to rotate about that point, transmitting a rotational force back through the saw and to the surgeon. This kicking or grabbing that occurs causes a loss of accuracy in the cut from the sudden, unpredictable movements of the saw and induces increased fatigue by the surgeon because of the greater tension that the surgeon must maintain in his hands and arms in anticipation of receiving this kicking or grabbing motion.
Another problem noted in existing blades involves the tendency of the saw to initially wander rather than to form a kerf. One reason for this involves the nature of oscillatory cutter blades. The handle portion of the saw remains stable because it is near the surgeon's control and remote from the cutting. However, a blade (having a proximal end mounted into the oscillatory power tool) moves at a distal end that scribes an arc of a circle. Most surgical cutting saw blades have teeth on the distal extremity which are also oriented in an arc of constant radius. Especially when that arc has a geometrical center which coincides with the center of oscillatory motion, the bone to be penetrated is normally initially addressed by only one active cutting tooth in the series of teeth that resides on the arc, and as the blade completes its outward motion many teeth come into contact simultaneously. With several teeth contacting the bone, there is a greater tendency of the saw blade to kick and wander. The effect is even more pronounced when the blade's center of the arc of its teeth is between the oscillatory center of the power tool and the blade's distal end.
Another problem involves the non-aggressive nature of prior art blades. Even when more than one tooth contacts the bone, it is primarily due to the manipulation of the blade by the surgeon. The effect is that adjacent teeth do not effect progressive cutting and therefore make binding and kick back more likely.
Moreover, once the kerf has been formed, teeth doing the work in penetrating the bone, several other problems attend the cutting process. For one thing, substantially all the saw's teeth traverse along the entire extent, the cutting action is not as aggressive as it could be since the effect is an abrading one rather than chipping. Those teeth reside within the kerf for a longer period of time and tend to naturally generate more heat than had they been allowed outside the kerf. In addition, not having the teeth exit the working surface periodically tends to leave the chips of bone that have been abraded by the teeth to remain between the teeth. Lack of efficient chip removal is recognized as one cause of excessive heat generation. In surgical situations, such unwanted heat generation is undesirable because of thermal necrosis which damages bone structure adjacent to the cut.
In our previously-filed U.S. patent application Ser. No. 07/707,903, we proposed a surgical blade to overcome the foregoing problems. Unlike prior art cutting blades, the cutting blade proposed in the patent application Ser. No. 07/707,903 has teeth substantially on a tangent which is perpendicular to the longitudinal axis of the cutting blade. It has been found that such a cutting blade minimizes "kick," minimizes the initial tendency of the blade to wander, and minimizes heat build-up within the kerf.
It has now been discovered that an additional advantage of the cutting blade disclosed in the patent application Ser. No. 07/707,903 is that its cutting efficiency is not diminished by changing the length of the blade. This is contrasted with prior art cutting blades having teeth disposed on an arc. Such prior art blades are only efficient when the radius of the arc along which the teeth are disposed is the same as the distance between the teeth and the oscillating connection to the saw. The practical consequence of this fact is that a surgical hospital has to maintain an inventory of several different prior art cutting blades of any one particular style to accommodate its staff surgeons who prefer cutting blades of different lengths.
As noted above, however, the cutting efficiency of the blade disclosed in the patent application Ser. No. 07/707,903 is not effected by changes in the length of the blade. Accordingly, if the cutting blade of the patent application Ser. No. 07/707,903 could be modified to be adjustably attachable to a surgical saw at various lengths, a surgical hospital need retain only one particular style cutting blade in inventory.
Accordingly, there is a need for a surgical cutting blade which can be adjustably installed within a surgical saw so that the cutting blade can be used at various lengths without diminishing its cutting efficiency. There is also a need for a surgical saw and blade combination in which the cutting blade is adjustable in length without diminishing the cutting efficiency of the combination.